[SystemZ] Improve instruction selection of 64 bit shifts and rotates.

For shift and rotate instructions that only use the last 6 bits of the shift
amount, a shift amount of (x*64-s) can be substituted with (-s). This saves
one instruction and a register:

  lhi     %r1, 64
  sr      %r1, %r3
  sllg    %r2, %r2, 0(%r1)
  =>
  lcr     %r1, %r3
  sllg    %r2, %r2, 0(%r1)

Review: Ulrich Weigand
llvm-svn: 357481